Measuring hydrocarbon content of a rock formation downhole using laser-induced vaporization and pyrolysis

ABSTRACT

A downhole tool to make one or more downhole measurements of laser-induced vaporization and/or pyrolysis of hydrocarbons is provided and disposed at a desired location within a wellbore. A tool head of the downhole tool is brought into sealing engagement with the wellbore wall. The fluid within an interior region enclosed by the tool head and the wellbore wall is evacuated and a measurement spot is irradiated with a laser to generate volatile hydrocarbons and/or pyrolytic hydrocarbons. Measurements are made on the volatile hydrocarbons and/or pyrolytic hydrocarbons and one or more formation properties are inferred based on the measurements. A low level of laser radiation intensity, irradiating some or all of the wellbore wall enclosing the interior region, may be used to prevent measurement contamination, and both medium power and high power levels of laser radiation may be used to first vaporize and then pyrolyze the hydrocarbons.

BACKGROUND

Total organic carbon (TOC) is a commonly sought property of ahydrocarbon-bearing subsurface formation. In recent years the level ofinterest in the measurement of this property has increased even furtherwith the emergence of shale oil and shale gas exploration andproduction. The shale formations being explored are typically morecomplex than conventional reservoir formations and they pose many morechallenges in their petrophysical studies and interpretations. Many ofthe standard measurement techniques commonly used in conventionalformations, such as measuring the TOC, do not work in shale.

Shale formations are highly laminated and their depositional historiesand transformation processes generally vary. The lamination thickness isnot constant, but rather may vary anywhere in the range of millimetresto meters. As a result, higher resolution measurements with shortspacings between the sampling points is important for evaluating theshales and to ensure any decision on the quality and economic potentialof the formation reflects the real system.

Laser induced pyrolysis (LIP) has been used to make certain formationevaluation measurements uphole, at the surface. For example, LIP hasbeen applied to core samples. However, that method may only be used if awell is cored, which is not particularly common. LIP may also be used onrock cuttings flushed to the surface while drilling. However, onegenerally has no idea of the depth within the well from which thecutting came. That is, during drilling operations pieces of rock are cutand brought to the surface by the circulating drilling fluid (mud).While the mud travels to the surface, it experiences turbulent flow,causing the cuttings to mix and their relative depth information to belost. In relatively homogeneous formations, measurements at the surfacemay succeed. However, shale cuttings, with their associated variablelaminations, should not be considered to be from a homogeneousformation. A LIP measurement on a cutting may provide a high resolutionmap of the lamination of that cutting, albeit with uncertain depthinformation, but the obtained lamination map is generally notrepresentative of the lamination of the shale reservoir.

SUMMARY

A downhole tool to make one or more downhole measurements oflaser-induced vaporization and/or pyrolysis of hydrocarbons is providedand disposed at a desired location within a wellbore. A tool head of thedownhole tool is brought into sealing engagement with the wellbore wall.The fluid within an interior region enclosed by the tool head and thewellbore wall is evacuated and a measurement spot is irradiated with alaser to generate volatile hydrocarbons and/or pyrolytic hydrocarbons.Measurements are made on the volatile hydrocarbons and/or pyrolytichydrocarbons and one or more formation properties are inferred based onthe measurements. A low level of laser radiation intensity, irradiatingsome or all of the wellbore wall enclosing the interior region, may beused to prevent measurement contamination, and both medium power andhigh power levels of laser radiation may be used to first vaporize andthen pyrolyze the hydrocarbons.

This summary is provided to introduce a selection of concepts that arefurther described below in the detailed description. This summary is notintended to identify key or essential features of the claimed subjectmatter, nor is it intended to be used as an aid in limiting the scope ofthe claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale. In fact, the dimensions of the variousfeatures may be arbitrarily increased or reduced for clarity ofdiscussion. Embodiments are described with reference to the followingfigures. The same numbers are generally used throughout the figures toreference like features and components.

FIG. 1 is a schematic drawing of one embodiment of a laser-inducedvaporization and pyrolysis measurement apparatus deployed in a wellbore,in accordance with the present disclosure.

FIG. 2 is a schematic drawing of one embodiment of the measurement headof the laser-induced vaporization and pyrolysis measurement apparatus ofFIG. 1, in accordance with the present disclosure.

FIG. 3 s a schematic drawing of one embodiment of the measurement deviceof the laser-induced vaporization and pyrolysis measurement apparatus ofFIG. 1, in accordance with the present disclosure.

FIG. 4 is a workflow showing an embodiment of making measurements usingthe laser-induced vaporization and pyrolysis measurement apparatus ofFIG. 1, in accordance with the present disclosure.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent embodiments, or examples, for implementing different featuresof various embodiments. Specific examples of components and arrangementsare described below to simplify the present disclosure. These are, ofcourse, merely examples and are not intended to be limiting. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.Moreover, the formation of a first feature over or on a second featurein the description that follows may include embodiments in which thefirst and second features are formed in direct contact, and may alsoinclude embodiments in which additional features may be formedinterposing the first and second features, such that the first andsecond features may not be in direct contact.

Some embodiments will now be described with reference to the figures.Like elements in the various figures may be referenced with like numbersfor consistency. In the following description, numerous details are setforth to provide an understanding of various embodiments and/orfeatures. However, it will be understood by those skilled in the artthat some embodiments may be practiced without many of these details andthat numerous variations or modifications from the described embodimentsare possible. As used herein, the terms “above” and “below,” “up” and“down,” “upper” and “lower,” “upwardly” and “downwardly,” and other liketerms indicating relative positions above or below a given point orelement are used in this description to more clearly describe certainembodiments. However, when applied to equipment and methods for use inwells that are deviated or horizontal, such terms may refer to a left toright, right to left, or diagonal relationship, as appropriate. It willalso be understood that, although the terms first, second, etc., may beused herein to describe various elements, these elements should not belimited by these terms. These terms are only used to distinguish oneelement from another.

The terminology used in the description herein is for the purpose ofdescribing particular embodiments only and is not intended to belimiting. As used in the description and the appended claims, thesingular forms “a,” “an” and “the” are intended to include the pluralforms as well, unless the context clearly indicates otherwise. It willalso be understood that the term “and/or” as used herein refers to andencompasses any and all possible combinations of one or more of theassociated listed items. It will be further understood that the terms“includes,” “including,” “comprises,” and/or “comprising,” when used inthis specification, specify the presence of stated features, integers,steps, operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, components, and/or groups thereof.

As used herein, the term “if” may be construed to mean “when” or “upon”or “in response to determining” or “in response to detecting,” dependingon the context. Similarly, the phrase “if it is determined” or “if [astated condition or event] is detected” may be construed to mean “upondetermining” or “in response to determining” or “upon detecting [thestated condition or event]” or “in response to detecting [the statedcondition or event],” depending on the context.

A system and method to measure the hydrocarbon content of a rockformation at a downhole location is disclosed. A specially designedapparatus capable of performing the operation downhole is used.Performing the measurements downhole avoids complications such asunwanted evaporation or drying of the core or other “live” sample versus“dead” sample issues that may arise when a core or cutting is brought tothe surface. A (medium power) laser beam may be used to cause the morevolatile hydrocarbons in a formation to vaporize (i.e., evaporate orboil). The vaporized hydrocarbons are pumped into a measurement chamberin the tool and measured using a plurality of techniques. The laserpower may then be increased, causing a higher local temperature that inturn causes the heavier, less volatile hydrocarbons to undergopyrolysis, thereby producing smaller, more volatile compounds (pyrolytichydrocarbons) that can be pumped into the tool and quantified.Performing the vaporization and pyrolysis and making the measurementsdownhole avoids the problem of relating the sample to the depth fromwhich it was generated. Because the LIP measurement is made downhole,the sequence of measurements is known and the distances between adjacentmeasurement points are well characterized. Also, because the LIPmeasurement is made downhole, a processor carried on the tool may beused to control operation of the tool and to store information inmemory.

Pyrolysis uses thermal energy to raise the temperature of a sample. Atrelatively lower temperatures (up to about 300° C.), medium sizemolecules vaporize and can be detected. At higher temperatures, the heatcauses the larger molecules, which are not volatile at any (reasonablyachievable) temperature (given the typical ambient pressures), toundergo cracking by breaking some of their bonds. This causes the largermolecule to break into a few smaller, more volatile, molecules that canbe vaporized and be detected.

If the source of thermal energy is a laser, then the process is calledLIP. The laser is usually chosen to be in the infrared wavelength rangebecause in this wavelength range the optical energy converts more easilyto heat than at other wavelengths. Lasers in the power range of tens ofwatts are readily available from many vendors and may be appropriate forthis application. These units are small to be readily incorporated intoa downhole tool without any need to modify the laser itself Lasers withmore power are also commercially available and can be used for thisapplication if they are engineered to fit within the space requirementsof a downhole tool. Infrared lasers are commonly used for metal welding,which requires much more power than is generally needed to crack a largehydrocarbon molecule. A 50-watt laser, for example, may focus the lightenergy in a spot size of about 1 mm by 1 mm. The power density in thespot is equivalent to 5000 watts/cm² or 50 megawatts/m², which is morepower than the 20 megawatts/m² that has been used successfully toperform uphole LIP.

The laser beam can be brought close to the target point using opticalfiber as is common in the art. If desired, using a lens with appropriatefocal length, the beam can also be focused to smaller cross sections,thereby increasing the power density. The focusing increases the powerdensity at the expense of a smaller spot size.

A downhole LIP tool 100 is shown in FIG. 1. Tool 100 may be part of alarger string of tools 150 depending on the application, and is equippedwith a head 110 that contacts and hydraulically seals against theformation or borehole wall 120. Since the borehole is usually filledwith drilling fluid that can interfere with LIP operations, a small spoton the wall is isolated from the borehole environment by pressing thehead 110 against the borehole wall. Hydraulically isolating a smallsection of the borehole wall is routinely done by the sampling toolsthat seal against the wall and pump a fluid sample out of the formation.In some applications, such as a wireline tool, a bow spring 130 may beused to press the head 110 (which may be equipped, for example, with anelastic (rubber) sealing material on its edge) firmly against theborehole wall 120. Alternatively, tool head 110 may be pressed againstthe borehole wall 120 using hydraulically energized members on theopposite side (i.e., opposite to head 110) of tool 100, as is common insampling tools. In LWD applications, tool 100 is part of a drill string,and a pressing mechanism similar to bow spring 130 may be used, butgenerally with a larger spring force so that the drill string (or atleast a portion of it) can be decentered in the wellbore and head 110pressed against borehole wall 120. In another embodiment, head 110 maybe telescopic and can be pressed against the borehole wall 120 withoutthe need to decenter tool 100. In yet another embodiment, the head 110may be part of a spring loaded pad (as is common in wireline tools) thatis normally closed but can be released to extend radially and pressagainst the borehole wall. In yet another embodiment, the head 110 maybe located in a space between two packers that are used to isolate asection of the borehole.

Head 110 is shown in further detail in FIG. 2. An elastic ring 210 isattached to the outer face of head 110 that, when pressed against theborehole wall 120, helps to form a hydraulic seal. The body 230 of head110 can be designed to be telescopic for particular implementations inwhich the tool 100 does not decenter in the borehole. Note the isolatedarea 250 is usually larger than the size of the measurement spot 260. Alaser 290 is provided in the body of tool 100 and is capable ofdelivering the required optical power. An optical fiber 220 delivers thelaser light to the measurement spot 260. The area of the measurementspot 260 is used to define the resolution of the LIP measurement.

Once head 110 is in contact with the borehole wall and hydraulicisolation is established, a pump 260, residing in tool 100, is used toremove any borehole fluid and possibly solid particles from the volume270 between the borehole wall 120 and the tool 100 within the regionisolated by head 110. While the volume 270 contains borehole fluid, thevalve 330 (shown in FIG. 3) is actuated to direct the fluid into pump260. Pump 260 removes the undesired fluid and discharges it back to theborehole using an exhaust pipe (port) and valve 280. Once the volume 270is empty, LIP operations can commence.

Applying thermal power (via the laser) to the formation generatesvolatile hydrocarbons that may be pumped away from spot 260 and intotool 100 for measurement. Once the borehole fluid is removed and laser290 is activated, valve 330 is actuated to redirect the volatilehydrocarbons to measurement system 350. A second pump 340 is activatedto facilitate the flow into measurement system 350. In other embodimentspump 260 can be used for both stages, eliminating the need for secondpump 340. Measurement system 350 comprises a solid separator 310, suchas an electrostatic solid separator, and a measurement module 320. Theelectrostatic solid separator 310 uses electrostatic energy to chargethe solid particles, if any, and absorb them in a capacitor, therebyremoving them from the gas stream. Alternatively, a physical filtrationsystem such as a screen (i.e., tight mesh or tightly packed particles)could be used. It is a good practice to remove those solids since thehigh power laser may cause some of the still solid hydrocarbons and rocksolids to vaporize and interfere with the measurements.

The measurement module 320 in measurement system 350 is used to measurethe volatile hydrocarbons. Different techniques may be used to measurethe volatile hydrocarbons. For example, one may use the techniquestraditionally used in mass spectrometry. In an embodiment in which amass spectrometer is used, a small fraction of the gas is delivered tothe mass spectrometer, which ionizes the delivered gas and then sendsthe ionized gas through a mass filter. The mass filter blocks all ionsexcept the specific component(s) of the mixture having a selected mass(i.e., m/z where z is the charge). The ions that pass through the massfilter impinge on a detector that provides the intensity or a numberproportional to the number of ions. The mass selection is varied and ateach selection the mass intensity is recorded, leading to a spectrumthat can be analyzed to identify and determine the concentration ofindividual hydrocarbons in the mixture. Use of a mass spectrometer fordownhole applications has been suggested previously in U.S. Pat. No.7,458,257.

In some applications, the detailed information about the components ofthe gas may not be needed. For example, only the total organic (i.e.,hydrocarbon) content (TOC) may be of interest. In these cases there isno need to identify the individual components—only the total carboncontent is determined. For such applications a further embodimentprovides a photoionization detector (PID) as part of the measurementmodule 320. The PID uses high energy photons such as ultravioletradiation to ionize the sample, but instead of measuring the ions basedon their mass (as was done by the mass spectrometer), the totalconductivity of the ionized sample is measured and used as an indicationof the number of ions. Thus, a current is injected into the chamberwhere the ions are present and an ampere meter is used to measure howmuch current passes through the chamber. The higher the number of ions,the more current can pass through and this proportionality is used toinfer the concentration of ions.

In another embodiment, a thermal conductivity detector (TCD) is used tomeasure the hydrocarbon gas. This is another detector traditionally usedin mass spectroscopy, but it can also be used as an independent device.The device is equipped with a heating solenoid and a thermal detector.In the absence of any gas in the detector, there is a backgrounddetector signal. Once gas is introduced into the detector, the thermalconductivity of the space between the heater and the detector changes,which causes a different detector signal level that can be calibratedand used to measure the relative concentrations of hydrocarbons in thegas.

The amount of laser power applied can be programmed as needed. Inup-hole applications, it is common to first use a low power levelillumination to remove any hydrocarbons that may be on the samplesurface. That is followed by a high power level illumination that isused to vaporize the volatile hydrocarbons and pyrolyze the heavierhydrocarbons up to some depth of penetration into the sample. Fordownhole applications, it may be desirable to apply the laser power inthree processes. In a first process, a low power laser pulse is used toclean the formation surface, as above. This may be needed since theborehole environment may contain hydrocarbons from other sources notrelated to the formation (such as oil base drilling fluid). In a secondprocess, a medium power level illumination is used that penetrates intothe formation and heats the volatile hydrocarbons (as used herein,“volatile hydrocarbons” means hydrocarbons that can be evaporated orboiled without being pyrolyzed). In a third process, the laser is usedat high power levels that penetrate the formation and crack (pyrolyze)the larger hydrocarbons.

FIG. 4 shows a workflow of an embodiment to make a downhole measurementof laser-induced vaporization and pyrolysis, in accordance with thisdisclosure. Tool 100 is brought to a depth of interest in the wellboreand tool head 110 is pressed (i.e., sealingly engaged) against boreholewall 120 (410). The wellbore depth may be determined, for example, usingan accompanying gamma ray tool (not shown). Next, the borehole fluidtrapped within the interior region of tool head 110 is pumped out (usingpump 340) and the surface is made ready for laser application (420).Upon completing evacuation of the interior region, the fluid flow pathis re-directed to measurement system 350 and pumping continues (430). Ifdesired, by-pass tube 360 and/or by-pass tube 370 may be used to divertthe gas and solid from entering measurement module 320. The laserradiation is initially delivered to the formation at the first (low)power level to evaporate and remove any contamination (e.g., that causedby the hydrocarbons normally mixed with the drilling fluid) at theborehole wall 120 (440). The laser radiation with medium power is thendelivered to the formation, causing the volatile hydrocarbons withinsome portion of the formation to evaporate or boil (i.e., vaporize) andbe measured (450). The laser radiation with high power is then deliveredto the formation, causing pyrolysis of at least some of the residualhydrocarbons in the formation (460). All those measurements may becombined to calculate the detailed hydrocarbon content information,which may be a detailed list of hydrocarbons present before pyrolysisand as a result of pyrolysis (470). Alternatively, only the totalhydrocarbon (organic) content (TOC) may be calculated and provided ifthat is the measurement objective.

Some of the methods and processes described above, including processes,as listed above, can be performed by a processor. The term “processor”should not be construed to limit the embodiments disclosed herein to anyparticular device type or system. The processor may include a computersystem. The computer system may also include a computer processor (e.g.,a microprocessor, microcontroller, digital signal processor, or generalpurpose computer) for executing any of the methods and processesdescribed above.

The computer system may further include a memory such as a semiconductormemory device (e.g., a RAM, ROM, PROM, EEPROM, or Flash-ProgrammableRAM), a magnetic memory device (e.g., a diskette or fixed disk), anoptical memory device (e.g., a CD-ROM), a PC card (e.g., PCMCIA card),or other memory device.

Some of the methods and processes described above can be implemented ascomputer program logic for use with the computer processor. The computerprogram logic may be embodied in various forms, including a source codeform or a computer executable form. Source code may include a series ofcomputer program instructions in a variety of programming languages(e.g., an object code, an assembly language, or a high-level languagesuch as C, C++, or JAVA). Such computer instructions can be stored in anon-transitory computer readable medium (e.g., memory) and executed bythe computer processor. The computer instructions may be distributed inany form as a removable storage medium with accompanying printed orelectronic documentation (e.g., shrink wrapped software), preloaded witha computer system (e.g., on system ROM or fixed disk), or distributedfrom a server or electronic bulletin board over a communication system(e.g., the Internet or World Wide Web).

Alternatively or additionally, the processor may include discreteelectronic components coupled to a printed circuit board, integratedcircuitry (e.g., Application Specific Integrated Circuits (ASIC)),and/or programmable logic devices (e.g., a Field Programmable GateArrays (FPGA)). Any of the methods and processes described above can beimplemented using such logic devices.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the scope of the present disclosure,and that they may make various changes, substitutions, and alterationsherein without departing from the scope of the present disclosure.

The Abstract at the end of this disclosure is provided to comply with 37C.F.R. §1.72(b) to allow the reader to quickly ascertain the nature ofthe technical disclosure. It is submitted with the understanding that itwill not be used to interpret or limit the scope or meaning of theclaims.

While only certain embodiments have been set forth, alternatives andmodifications will be apparent from the above description to thoseskilled in the art. These and other alternatives are consideredequivalents and within the scope of this disclosure and the appendedclaims. Although only a few example embodiments have been described indetail above, those skilled in the art will readily appreciate that manymodifications are possible in the example embodiments without materiallydeparting from this invention. Accordingly, all such modifications areintended to be included within the scope of this disclosure as definedin the following claims. In the claims, means-plus-function clauses areintended to cover the structures described herein as performing therecited function and not only structural equivalents, but alsoequivalent structures. Thus, although a nail and a screw may not bestructural equivalents in that a nail employs a cylindrical surface tosecure wooden parts together, whereas a screw employs a helical surface,in the environment of fastening wooden parts, a nail and a screw may beequivalent structures. It is the express intention of the applicant notto invoke 35 U.S.C. §112, paragraph 6 for any limitations of any of theclaims herein, except for those in which the claim expressly uses thewords ‘means for’ together with an associated function.

What is claimed is:
 1. A method, comprising: providing a downhole toolto make one or more downhole measurements of laser-induced vaporizationand/or pyrolysis of hydrocarbons, and disposing the downhole tool at adesired location within a wellbore; bringing a tool head of the downholetool into sealing engagement with a wall of the wellbore; evacuatingfluid within an interior region enclosed by the tool head and thewellbore wall; irradiating a measurement spot with a laser to generatevolatile hydrocarbons and/or pyrolytic hydrocarbons; making measurementson the volatile hydrocarbons and/or pyrolytic hydrocarbons; andinferring one or more formation properties based on the measurements. 2.The method of claim 1, further comprising irradiating with a low levelof laser radiation some or all of the wellbore wall enclosing theinterior region.
 3. The method of claim 2, wherein irradiating ameasurement spot with a laser comprises irradiating with a medium powerlevel of laser radiation some depth into a formation.
 4. The method ofclaim 3, wherein irradiating a measurement spot with a laser furthercomprises irradiating with a high power level of laser radiation somedepth into the formation.
 5. The method of claim 1, wherein thegenerated volatile hydrocarbons and/or pyrolytic hydrocarbons aredirected into a measurement device.
 6. The method of claim 1, furthercomprising filtering solid particles from the generated volatilehydrocarbons and pyrolytic hydrocarbons.
 7. The method of claim 6,wherein the filtering uses a device selected from the group consistingof an electrostatic solid separator, a screen, and tightly packedparticles.
 8. The method of claim 1, wherein the measurements are madeusing a device selected from the group consisting of a massspectrometer, a photoionization detector, and a thermal conductivitydetector.
 9. The method of claim 1, wherein the one or more formationproperties are selected from the group consisting of detailedhydrocarbon content and total organic content.
 10. The method of claim1, wherein bringing the tool head of the downhole tool into sealingengagement with a wall of the wellbore comprises using a biasing deviceor a telescoping device.
 11. The method of claim 1, further comprisingfocusing the laser radiation onto a smaller area using a lens.
 12. Themethod of claim 1, wherein the irradiating further comprises using oneor more optical fibers to deliver the laser radiation to the measurementspot.
 13. A non-transitory, computer-readable storage medium, which hasstored therein one or more programs, the one or more programs comprisinginstructions, which when executed by a processor, cause the processor toperform a method comprising: bringing a tool head of the downhole toolinto sealing engagement with a wall of the wellbore; evacuating fluidwithin an interior region enclosed by the tool head and the wellborewall; irradiating a measurement spot with a laser to generate volatilehydrocarbons and/or pyrolytic hydrocarbons; making measurements on thevolatile hydrocarbons and/or pyrolytic hydrocarbons; and inferring oneor more formation properties based on the measurements.
 14. The storagemedium of claim 13, further comprising irradiating with a low level oflaser radiation some or all of the wellbore wall enclosing the interiorregion, and wherein the irradiating a measurement spot with a lasercomprises irradiating first with a medium power level of laser radiationand then with a high power level of laser radiation.